Testing of semiconductor chips with microbumps

ABSTRACT

A test structure including an array of microbumps electrically connecting a chip and a substrate, wherein a width of each microbump of the array of microbumps is equal to or less than about 50 microns (μm). The test structure further includes an interconnect structure connected to the array of microbumps. The test structure further includes an array of test pads around a periphery of the array of microbumps, wherein a test pad of the array of test pads is connected to a corresponding microbump of the array of microbumps through the interconnect structure. A width of the test pad is greater than a width of the corresponding microbump, and the test pad is adapted to be covered after circuit probing by a passivation material to prevent particle and corrosion issues.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No.13/025,931, filed Feb. 11, 2011, the disclosure of which is incorporatedherein by reference in its entirety.

FIELD

This disclosure relates generally to integrated circuits, and moreparticularly to testing of semiconductor chips with microbumps.

BACKGROUND

Integrated circuits have experienced continuous rapid growth due toconstant improvements in the integration density of various electroniccomponents (i.e., transistors, diodes, resistors, capacitors, etc.). Forthe most part, this improvement in integration density has come fromrepeated reductions in minimum feature size, allowing more components tobe integrated into a given chip area.

The volume occupied by the integrated components is near the surface ofthe semiconductor wafer. Although dramatic improvements in lithographyhave resulted in considerable improvements in two-dimensional (2D)integrated circuit formation, there are physical limitations to thedensity that can be achieved in two dimensions. One of these limitationsis the minimum size needed to make these components. Further, when moredevices are put into one chip, more complex designs are required. Anadditional limitation comes from the significant gains in the number andlength of interconnections between devices as the number of devicesincreases. When the number and length of interconnections increase, bothcircuit resistive-capacitive (RC) delay and power consumption increase.

Three-dimensional integrated circuits (3DIC) were thus formed to addressissues raised by increase in circuit densities. The dies are stacked,with wire-bonding, flip-chip bonding, and/or through-silicon vias (TSV)being used to stack the dies together and to connect the dies to packagesubstrates. Circuit probe (CP) testing of advanced semiconductor chipsand 3DIC with high device density becomes challenging.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be readily understood by the followingdetailed description in conjunction with the accompanying drawings, andlike reference numerals designate like structural elements.

FIG. 1 shows a schematic diagram of an integrated circuit (IC) chip on apackage substrate, in accordance with some embodiments.

FIG. 2 shows a top schematic view of a portion of a semiconductor chipwith bumps, in accordance with some embodiments.

FIG. 3 shows a schematic diagram of a packaged semiconductor chipinvolving microbumps, in accordance with some embodiments.

FIG. 4A shows a top view of a number of microbumps surrounding a testpad, in accordance with some embodiments.

FIG. 4B shows examples of shapes of the testing pads described above, inaccordance with some embodiments.

FIGS. 4C (a)-(f) show a top view of a number of configurations ofmicrobumps with test pads, in accordance with some embodiments.

FIGS. 4D (a)-((f) show a top view of a number of configurations ofmicrobumps with test pads, in accordance with some other embodiments.

FIGS. 5A-5C are cross-sectional views of a region of a testing pad in aprocess of probing and formation of microbumps, in accordance with someembodiments.

FIG. 6A shows a bump structure on a substrate, in accordance with someembodiments.

FIG. 6B shows a copper post on a substrate, in accordance with someembodiments.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different features.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. In addition, the present disclosuremay repeat reference numerals and/or letters in the various examples.This repetition is for the purpose of simplicity and clarity and doesnot in itself dictate a relationship between the various embodimentsand/or configurations discussed.

Modern integrated circuits are made of millions of active devices, suchas transistors and capacitors. These devices are initially isolated fromeach other, but are later interconnected together to form functionalcircuits. Typical interconnect structures include lateralinterconnections, such as metal lines (wirings), and verticalinterconnections, such as vias and contacts. Interconnections areincreasingly determining the limits of performance and the density ofmodern integrated circuits. On top of the interconnect structures, bondpads are formed and exposed on the surface of the respective chip.Electrical connections are made through these bond pads to connect thechip to a package substrate or another die. Bond pads can be used forwire bonding or flip-chip bonding. Flip-chip packaging utilizes bumps toestablish electrical contact between a chip's I/O pads and the substrateor lead frame of the package.

FIG. 1 shows a schematic diagram of an integrated circuit (IC) chip 100on a package substrate 110, in accordance with some embodiments. IC chip100 is packaged with flip-chip packaging by forming bumps 105 on thefront-side of the IC chip 100. Bumps 105 electrically and possiblyphysically contact the I/O pads (metal pads) 103 of the IC chip 100. Insome embodiments, there is an under bump metallurgy (UBM) layer 104between the bumps 105 and the metal pads 103. The IC chip 100 with thebumps 105 is flipped over to be placed on a package substrate 110. Thesurface of substrate 110 may have metal pads 107 to receive bumps 105.In some embodiments, the space between and around the IC chip 100, thebumps 105 and the substrate 100 could be filled with an under-fillmaterial (not shown). The embodiment shown in FIG. 1A is merely anexample. Other embodiments are also possible. IC chip 100 with bumps 105could be applied on other types of substrates, such as an applicationboard, and a substrate with embedded passive and/or active devices.

Conventionally, the sizes of the flip chip bumps are equal to or greaterthan about 75 μm. The sizes of the conventional flip chip bumps allowthe semiconductor chips connected to the bumps to be electrically testedto determine whether the circuits under (or connected) to the bumps passfunctional tests. Sometimes such as tests may also called circuit probe(CP), or electronic die sort (EDS).

Conventional bumps, such as bumps 105, can be probed for such tests.Alternatively, test pads can be added to allow functional tests. FIG. 2shows a top schematic view of a portion 200 of a semiconductor chip withbumps, in accordance with some embodiments. The metal pads (or bumppads) 201 under bumps are represented by octagons in FIG. 2. As shown inFIG. 1, bumps (not shown) may be placed directly above these metal pads.There are a number of test pads 202 surrounding the metal pads 201 (andbumps above them). The test pads are connected to the metal pads viaconductive lines 203. The conductive lines may be redistribution lines(RDLs), which are formed above the metal pads under the bumps, metallines, or post-passivation interconnects (PPI). The test pads 202 may beat the same or at different level as the conductive lines 203. The testpads 202 are connected to different bumps 201 to perform functionaltests. However, placing the test pads 202 around the bumps 201 takesaway the real-estate on the surface of the semiconductor chip.

As feature size scales down, the number of transistors and interconnectson a chip has increased continuously. As a result, the number of chip topackage input/output (I/O) interconnects have also increasedsignificantly. With the increased chip to package I/O interconnects, thesizes of bumps could be reduced to equal to or less than about 50 μm.Such reduced-size bumps are called “microbumps.”

FIG. 3 shows a schematic diagram of a packaged semiconductor chipinvolving microbumps, in accordance with some embodiments. Asemiconductor chip 300 (a flipped chip) is disposed on a siliconsubstrate 320, which has through silicon vias 325 for assisting heatdissipation. The silicon substrate 320 may or may not have activedevices on the substrate. The semiconductor chip 300 is connected tosilicon substrate 320 via microbumps 310. The silicon substrate 320 isplaced on another package substrate 340, with bumps 330, which areregular bumps and are larger than microbumps 310.

After semiconductor chip 300 is prepared, and before it is placed onsubstrate 320, it is desirable to perform electrical tests on chip 300to determine if the circuits on chip 300 meet the specification offunctional tests. If the test results show problems with chip 300, chip300 could be discarded and another good chip could be used instead. Suchscreening can prevent the entire package shown in FIG. 3 from beingdiscarded and can increase package yield, which would result incost-saving.

As mentioned above, conventional flip chip bumps, with sizes equal to orgreater than about 75 μm, allow circuit probing (or electrical tests).However, the current circuit probes are too large for microbumps (orμbumps). For example, the sizes (or widths) of the tips of probes may bein a range from about 2.5 mil to about 5 mil, which are too large formicrobumps. An alternative for testing semiconductor chips withmicrobumps would be to use test pads, such as those described in FIG. 2.However, test pads occupy the precious real-estate on the surface of thesemiconductor chip, as mentioned above.

As mentioned above, regular bumps are much larger than microbumps. Forexample, regular bumps have diameters in a range from about 75 μm toabout 150 μm. In contrast, the diameters of microbumps are in a rangefrom about 20 μm to about 50 μm, in accordance with some embodiments. Asa result, extra space on the surface of semiconductor may becomeavailable due to the usage of microbumps.

FIG. 4A shows a top view 400 of a number of bump pads 410 surrounding atest pad 420, in accordance with some embodiments. FIG. 4A shows 4 bumppads 410 surrounding the test pad 420. The bump pads 410 are connectedto test pad 420 via metal lines 430. Due to the smaller size of themicrobumps, the bump pads 410 are also smaller, with widths onlyslightly larger than the 20 μm to about 50 μm. The exemplary bump pads410 are shown to be in octagonal shape in FIG. 4A. Other shapes are alsopossible. The testing pad 420 is shown in FIG. 4A to be a square. Thetest pad 420 may also be in other shapes, such as a circle, a hexagon,an octagon, a rectangle, or other polygons. FIG. 4B shows examples ofshapes of the testing pads 420 described above, in accordance with someembodiments. The smallest width of the test pads 420 is greater thanabout 60 which allows probing tests, in accordance with someembodiments. The distance D between the bump pads 410 and the test pad420 is in a range from about 20 μm to about 80 μm.

Since the microbumps are smaller than regular bumps. The surface spacesaved from using microbumps, instead of using regular bumps, could beused for the testing pads. Using the surface saved by using microbumpsfor test pad(s) can minimize the impact of the test pads occupying thereal-estate of the surface of a semiconductor chip. FIG. 4A shows a padarea 450 for a regular bump, in accordance of some embodiments. Themicrobumps 410 and the testing pad 420 are within the pad area 450 of aregular bump. The microbumps 410 and the testing pad 420 could alsooccupy an area slightly larger than the pad area 450 of a regular bump.However, the usage of microbumps saves surface real-estate and canminimize the impact of having a test pad(s).

FIGS. 4C (a)-(f) show a top view of a number of configurations ofmicrobumps with test pads, in accordance with some embodiments. In FIGS.4C (a)-(f), the bump pads 461, 462, 463, 464, 465, and 466 surround thetest pads 471, 472, 473, 474, 475, and 476 and are distributedsymmetrically around the test pads. The probing areas are within thetest pads due to coverage of edge areas by a passivation layer.Similarly, the microbumps are placed within the bump pads. The bump padsand testing pads are connected to one another via conductive lines 491,492, 493, 494, 495, and 496, which are on the same level or differentlevel of the metal pads, 461-466 and 471-476. The testing pads areillustrated to be in octagonal shapes. However, they could be in othershapes, as described in FIG. 4B. The bump pads under the microbumps areshown in octagonal shapes, which are also merely exemplary.

The number of bump pads (or microbumps) could be in a range from 2 to 8,in accordance with some embodiments. For smaller microbumps, the numberof bumps can be even higher. Different numbers of microbumps could beconnected to the test pads to allow performing functional tests ofdifferent devices connected to the same I/O connections and/or under thesame input signals. For example, some functions of the semiconductorchip could involve applying or pulling signals (or current) from devicesconnected to a number of bumps. The different connections show in FIGS.4C (a)-(f) allow such testing. The symmetrical configuration of bumppads for microbumps could make stress distribution due to the formationof microbumps more even, which could reduce the chance of interfacedelamination for the microbumps.

Alternatively, the microbumps do not need to be distributedasymmetrically around the testing pads. FIGS. 4D (a)-(f) show a top viewof a number of configurations of microbumps with test pads, inaccordance with some other embodiments. FIG. 4D (a)-(f) show differentnumbers of microbumps are connected to the testing pads and are arrangedasymmetrically.

In some embodiments, there are a number of test pads with microbumps insome of the arrangements described above on a semiconductor chip.Different combinations could be needed on different chips to completefunctional testing of various devices on the chips. For example,different inputs/outputs (I/Os), such as I/Os for signals, power, andground (or grounding), need different numbers of microbumps due todifferent current requirement and also the I_(max) (maximum current)limits of microbumps. Therefore, different combinations of numbers ofmicrobumps are needed to allow testing.

Circuit probing can damage the metal pads, which leads to the copperseed layer coverage and poor bump plating (or formation). Poor bumpformation could lead to particle and corrosion issues. However, if thetesting pads are covered by a passivation layer after the testing iscompleted, the risks of such issues are completed resolved or greatlyreduced.

FIGS. 5A-5C are cross-sectional views of a region 500 of a testing padin a process of probing and formation of microbumps, in accordance withsome embodiments. FIG. 5A shows that a test pad region 510 and twoopening regions (or bump pads) 520 for forming microbumps. Under the twoopening regions (or bump pads) 520 for forming microbumps, there areinterconnects 523, 524 connecting the to-be-formed microbumps withdevices 525 and 526. The interconnect 523, 524 and devices 525, 526 (onsubstrate 505) shown in FIG. 5A are merely exemplary. Otherconfigurations and additional interconnects/devices could be involved.The test pad region 510 is formed on a metal pad 530, which could be atop metal layer, a redistribution layer (RDL), or a post passivationinterconnect (PPI) layer, in accordance with some embodiments. Regions520 and test pad region 510 are electrically and physically connected.FIG. 5A shows that the test pad region 510 and the regions 520 aredefined by a first passivation layer 540 through lithographicalpatterning. The first passivation layer 540 is deposited over the metalpad 530. The test pad region 510 is separated from the regions 520 sothat the microbumps formed in regions 520 would be physically separatedfrom the test pad 510. The outline(s) of the conductive line(s) betweentest pad and microbump pad(s) are also defined by the first passivationlayer, which is not shown in FIG. 5A.

FIG. 5A shows a probe 550 touching the test pad region 510. The probingcauses damage (see region 555) on the surface of the test pad region510. The probing in test pad region 510 allowed electrical data relatedto devices connected to regions 520 to be tested. As mentioned above,the regions for forming microbumps that are connected to the test padcould be in a range from 1 to many. After the probing is completed,microbumps will be formed on regions 520. In some other embodiments, anUBM layer (not shown) is formed after the probing is performed. The UBMlayer is under the microbumps to provide a diffusion barrier and toenhance adhesion. In some embodiments, the UBM layer (not shown) isformed to cover the openings 520 before probing is performed. Thestructure of microbumps with details of the UBM layer will be describedlater. The formation of the UBM layer involves deposition, patterningand etching the UBM layer. After the UBM layer is formed, the substratea second passivation layer 570 is deposited and patterned to exposeregions 520. The second passivation layer 570 may be made of an organicpolymer, such as polyimide. In some embodiments, the second passivationlayer 570 is made of a photosensitive polyimide. In some otherembodiments, the second passivation layer 570 may be a dielectricmaterial, which may be deposited by chemical vapor deposition or byspin-on. For example, the dielectric material may be SiO₂, SiN, or othersuitable passivation material. The test pad region 520 is covered by thesecond passivation layer 570; therefore, the issues such as particlesand corrosion due to test pad damage are resolved. The damaged area iscovered and protected. In some embodiments, the thickness of the secondpassivation layer 570 is in a range from about 1 μm to about 10 μm. Insome other embodiments, the thickness of the second passivation layer570 is in a range from about 2 μm to about 5 μm.

FIG. 5C shows the microbumps 580 formed on the substrate of FIG. 5Bafter the second passivation layer 570 is formed, in accordance withsome embodiments. The microbumps 580 may be formed by plating, inaccordance with some embodiments. Prior to plating the substrate, aphotoresist, which could be wet or dry (not shown in FIG. 5C), ispatterned over the second passivation layer 570 to define of themicrobumps 580 above regions 520. The microbumps can be made of variousmaterials, such as solder or copper. After the microbumps are deposited,substrate 505 may undergo a reflow process.

The structures of microbumps and the processes of forming the microbumpscould be similar to regular bumps. FIGS. 6A and 6B show two exemplarystructures of microbumps on substrates, in accordance with someembodiments. FIG. 6A shows a bump structure on a substrate 600, inaccordance with some embodiments. Metal pad 628, which is used as bumppad, is formed over one or more interconnect structures (not shown).Metal pad 628 may comprise aluminum, and hence may also be referred toas aluminum pad 628, although it may also be formed of, or include,other materials, such as copper, silver, gold, nickel, tungsten, alloysthereof, and/or multi-layers thereof. In some embodiments, a passivationlayer 630 is formed to cover edge portions of metal pad 628. Thepassivation layer 630 may be formed of polyimide or other knowndielectric materials. Additional passivation layers may be formed overthe interconnect structures (not shown) and at the same level, or over,metal pad 628.

An opening is formed in passivation layer 630, with metal pad 628exposed. A diffusion barrier layer 640 and a thin seed layer 642 areformed to cover the opening with the diffusion barrier layer 640 incontact with the metal pad 628, in accordance with some embodiments.Diffusion barrier layer 640 may be a titanium layer, a titanium nitridelayer, a tantalum layer, or a tantalum nitride layer. The materials ofseed layer 642 may include copper or copper alloys, and hence isreferred to as copper seed layer 642 hereinafter. However, other metals,such as silver, gold, aluminum, and combinations thereof, may also beincluded. In some embodiments, diffusion barrier layer 640 and copperseed layer 642 are formed using sputtering.

A copper layer 650 may be deposited or plated on the exposed surface ofcopper seed layer 642, in accordance with some embodiments. A metallayer 652 may be optionally formed on the copper layer 650. In someembodiments, metal layer 652 is a nickel-containing layer comprising,for example, a nickel layer or a nickel alloy layer by plating. A solderlayer 660 is formed on nickel layer 652, for example, by plating. Solderlayer 660 may be a lead-free pre-solder layer formed of, for example,SnAg, or a solder material, including alloys of tin, lead, silver,copper, nickel, bismuth, or combinations thereof. A solder reflowingprocess is performed to form solder bump 660 a, as shown in FIG. 6A.

In alternative embodiments, as shown in FIG. 6B, the thickness of copperlayer 650 is increased so that copper layer 650 becomes a copper post(or pillar). In some embodiments, after the optional formation of metallayer 652 on copper post 650, a solder layer 662, which may be a thinsolder layer, may be plated on metal layer 652. The embodiments shown inFIGS. 6A and 6B are only two examples; other embodiments of bumps arealso possible. Further details of bump formation process may be found inU.S. patent application Ser. No. 12/842,617, filed on Jul. 23, 2010 andentitled “Preventing UBM Oxidation in Bump Formation Processes,” andU.S. patent application Ser. No. 12/846,353, filed on Jul. 29, 2010 andentitled “Mechanisms for Forming Copper Pillar Bumps,” both of which areincorporated herein in their entireties.

The embodiments described above provide mechanisms for performingfunctional tests on devices connected to microbump pads undermicrobumps. Test pads that are larger than microbumps are formed toallow such testing. Due to the surface areas saved by using microbumps,the effect of test pads on surface real-estate of semiconductor chipscould be reduced to minimum or none. The test pad can be connected toone or more microbump pads during testing. These one or more microbumpsmay be distributed symmetrically or asymmetrically around the test pads.The test pads could be damaged due to circuit probing and could becovered by a passivation layer after probing to protect the damagedsurface.

An aspect of this description relates to a test structure including anarray of microbumps electrically connecting a chip and a substrate,wherein a width of each microbump of the array of microbumps is equal toor less than about 50 microns (μm). The test structure further includesan interconnect structure connected to the array of microbumps. The teststructure further includes an array of test pads around a periphery ofthe array of microbumps, wherein a test pad of the array of test pads isconnected to a corresponding microbump of the array of microbumpsthrough the interconnect structure. A width of the test pad is greaterthan a width of the corresponding microbump, and the test pad is adaptedto be covered after circuit probing by a passivation material to preventparticle and corrosion issues.

Another aspect of this description relates to a test structure includinga test pad having a first width. The test structure further includes afirst microbump electrically connected to the test pad, the firstmicrobump having a second width, wherein the first microbump isconfigured to provide a first electrical connection between a chip and asubstrate. The test structure further includes a second microbumpelectrically connected to the test pad through the first microbump, thesecond microbump having the second width, wherein the second microbumpis configured to provide a second electrical connection between the chipand the substrate, and the first width is greater than the second width.

Still another aspect of this description relates to a test structureincluding a test pad having a first width. The test structure furtherincludes a first microbump electrically connected to the test pad by ametal line, the first microbump having a second width, wherein the firstmicrobump is configured to provide a first electrical connection betweena chip and a substrate. The test structure further includes a secondmicrobump electrically connected to the test pad by the metal line, thesecond microbump having the second width, wherein the second microbumpis configured to provide a second electrical connection between the chipand the substrate, and the first width is greater than the second width.

Various modifications, changes, and variations apparent to those ofskill in the art may be made in the arrangement, operation, and detailsof the methods and systems disclosed. Although the foregoing embodimentshave been described in some detail for purposes of clarity ofunderstanding, it will be apparent that certain changes andmodifications may be practiced within the scope of the appended claims.Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the disclosure is not to belimited to the details given herein, but may be modified within thescope and equivalents of the appended claims.

What is claimed is:
 1. A test structure comprising: an array ofmicrobumps electrically connecting a chip and a substrate, wherein awidth of each microbump of the array of microbumps is equal to or lessthan about 50 microns (μm); an interconnect structure connected to thearray of microbumps; and an array of test pads around a periphery of thearray of microbumps, wherein a test pad of the array of test pads isconnected to a corresponding microbump of the array of microbumpsthrough the interconnect structure, wherein a width of the test pad isgreater than a width of the corresponding microbump, and the test pad isadapted to be covered after circuit probing by a passivation material toprevent particle and corrosion issues.
 2. The test structure of claim 1,wherein at least two test pads of the array of test pads are configuredto receive a test probe for testing an electrical connection between thechip and the substrate.
 3. The test structure of claim 1, wherein thesubstrate comprises through silicon vias.
 4. The test structure of claim1, wherein a first side of the test pad of the array of test pads isconnected to a first microbump of the array of microbumps, and a secondside of the test pad is connected to a second microbump of the array ofmicrobumps.
 5. The test structure of claim 4, wherein the first side isthe same as the second side.
 6. The test structure of claim 5, whereinthe second microbump is connected to the test pad through the firstmicrobump.
 7. The test structure of claim 4, wherein the first side isadjacent to the second side.
 8. The test structure of claim 4, whereinthe first side is opposite to the second side.
 9. The test structure ofclaim 1, wherein the test pad of the array of test pads is connected toa first microbump of the array of microbumps by a portion of theinterconnect structure comprising a first section and a second sectionperpendicular to the first section.
 10. The test structure of claim 1,wherein a distance from the test pad of the array of test pads to thecorresponding microbump of the array of microbumps ranges from about 20μm to about 80 μm.
 11. A test structure comprising: a test pad having afirst width; a first microbump electrically connected to the test pad,the first microbump having a second width, wherein the first microbumpis configured to provide a first electrical connection between a chipand a substrate; a second microbump electrically connected to the testpad through the first microbump, the second microbump having the secondwidth, wherein the second microbump is configured to provide a secondelectrical connection between the chip and the substrate, and the firstwidth is greater than the second width.
 12. The test structure of claim11, wherein at least one of the first microbump or the second microbumpcomprises a copper pillar.
 13. The test structure of claim 11, whereinat least one of the first microbump or the second microbump comprises asolder bump.
 14. The test structure of claim 11, wherein at least one ofthe first microbump or the second microbump comprises an under bumpmetallurgy (UBM) layer, wherein the UBM layer comprises: a diffusionbarrier layer; and a seed layer over the diffusion barrier layer. 15.The test structure of claim 14, wherein the UBM layer further comprisesa nickel-containing layer.
 16. A test structure comprising: a test padhaving a first width; a first microbump electrically connected to thetest pad by a metal line, the first microbump having a second width,wherein the first microbump is configured to provide a first electricalconnection between a chip and a substrate; a second microbumpelectrically connected to the test pad by the metal line, the secondmicrobump having the second width, wherein the second microbump isconfigured to provide a second electrical connection between the chipand the substrate, and the first width is greater than the second width.17. The test structure of claim 16, wherein at least one of the firstmicrobump or the second microbump comprises a copper layer.
 18. The teststructure of claim 17, wherein the at least one of the first microbumpor the second microbump comprises a solder bump over the copper layer.19. The test structure of claim 18, wherein the at least one of thefirst microbump or the second microbump comprises a nickel-containinglayer between the solder bump and the copper layer.
 20. The teststructure of claim 16, wherein the metal line comprises a first sectionand a second section, and the first section is perpendicular to thesecond section.